The present invention relates to piezoelectric transducer assemblies and methods for generating radiation patterns.
Piezoelectricity is pressure electricity and piezoelectric behavior is the characteristic of materials to deform upon the application of electrical signals or conversely to develop electricity whenever deformed by the application of pressure. Materials exhibiting piezoelectric behavior are naturally occurring or may be man made.
Heretofore, planar piezoelectric elements which vibrate at a natural resonant frequency have been employed in transducer assemblies, particularly ultrasonic transducer assemblies. When vibrating at their natural resonant frequency, such planar piezoelectric elements flex about a node defined thereon; the portion of the element on one side of the node always vibrating in a direction opposite to the direction of vibration of the portion of the element on the other side of the node. A planar piezoelectric element having this type of vibration generates or senses compression and rarefaction waves essentially in a direction perpendicular to its planar surface.
In order to increase the acoustical output of transducer assemblies employing such planar piezoelectric elements, structural arrangements have been devised which operate to cause the compression and rarefaction waves generated on opposite sides of the element node and also on opposite sides of the plane of the piezoelectric element to combine through constructive interference. As a consequence, a generally spherical radiation pattern is typically generated by these transducers directed from one planar side of the piezoelectric element along an axis perpendicular to its plane. In such a spherical radiation pattern, the energy level is at a maximum along the axis, decreasing first gradually then rapidly with increasing angular offset from the axis. For example, at a point 45.degree. off the axis, the energy level has decreased by approximately six decibels from the level on the axis. Thus, it is apparent that the spherical radiation patterns generated by these prior art transducer assemblies are basically unidirectional along the subject axis. Conversely, it is noted that these transducers when used to detect acoustical waves are equally more sensitive to waves received along this axis.
Transducers characterized by having essentially unidirectional radiation patterns can be advantageously employed in ultrasonic detection systems by mounting them in adjustable mounts since their axes of radiation may be set, and/or repositioned as necessary to effectively monitor an area to be protected.
In many instances, however, it is desirable to be able to control the shape of the radiation pattern of transducers particularly where it is desired to cover a long narrow hallway or isle on one hand or a large room area on the other hand.